A known apparatus comprises two current transformers connected to respective ends of the circuit to be protected. The secondary voltages from these current transformers are applied to respective inputs of differential relay.
An output signal from the relay indicates the presence of a fault situated between the two current transformers, i.e. between the circuit and ground, and the fault current is proportional to said signal.
Such apparatus does not detect faults within the circuit itself, such as a short circuit between turns of the inductance winding, or such as abnormal operation due to unwanted firing or lack of firing in the thyristors of the static switch feeding the inductance winding.
Preferred embodiments of the present invention mitigate these defects.